Movement and Age of Ground Water

in the Western Part of the Mojave Desert,

Southern California, USA

By John A. Izbicki and Robert L. Michel

Sacramento, California

2004

Water-Resources Investigations Report 03-4314

U.S. GEOLOGICAL SURVEY

Prepared in cooperation with the

MOJAVE WATER AGENCY 7002-06
U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does 

not imply endorsement by the U.S. Government.

For additional information write to:

District Chief

U.S. Geological Survey

Placer Hall, Suite 2012

6000 J Street

Sacramento, CA 95819-6129

http://ca.water.usgs.gov
Contents iii

CONTENTS

Abstract ................................................................................................................................................................ 1

Introduction .......................................................................................................................................................... 1

Purpose and Scope ...................................................................................................................................... 1

Background and Previous Studies............................................................................................................... 1

Tritium ............................................................................................................................................... 1

Carbon-14 and Carbon-13.................................................................................................................. 3

Sample Collection and Analyses................................................................................................................. 4

Acknowledgments....................................................................................................................................... 4

Hydrogeology....................................................................................................................................................... 5

Mojave River Ground-Water Basin ............................................................................................................ 5

Morongo Ground-Water Basin ................................................................................................................... 6

Tritium.................................................................................................................................................................. 6

Carbon-14 and Carbon-13.................................................................................................................................... 8

Areal Distribution of Carbon-14 Activities................................................................................................. 8

Carbon-14 Activities Along Selected Geologic Sections in the Mojave River Basin ................................ 10

Sections A–A', B–B', and C–C’ .......................................................................................................... 10

Sections D–D' and E–E'..................................................................................................................... 14

Interpretation of Ground-Water Age Along Selected Flow Paths .............................................................. 14

Initial Carbon-14 Activity.................................................................................................................. 15

Geochemical Reactions...................................................................................................................... 17

Interpretation of Carbon-14 Ages Along Selected Flow Paths.......................................................... 18

Flow Path 1: Surprise Spring Basin.......................................................................................... 18

Flow Path 2: Upper Part of the Mojave River Basin to the East of the Mojave River ............. 24

Flow Path 3: Upper Part of the Mojave River Basin to the West of the Mojave River............ 27

Limitations on Carbon-14 Interpretations.......................................................................................... 28

Discussion ............................................................................................................................................................ 28

References ............................................................................................................................................................ 33
iv Figures

FIGURES

Figure 1. Map showing location of study area, the Mojave River and the Morongo groundwater basins, the floodplain aquifer, selected faults, and geologic section lines............................... 2

Figure 2. Map showing tritium in water from wells in the Mojave River and the Morongo 

ground-water basins, western Mojave Desert, southern California, 1992–2000............................... 7

Figure 3. Diagrams showing carbon-14 activity in water from wells in the Mojave River and

the Morongo ground-water basins, western Mojave Desert, southern California, 1992–2000 ......... 9

Figure 4. Diagram showing delta-deuterium composition and carbon-14 activity of water from

selected observation wells along Section A–A', Mojave River ground-water basin, 

southern California ............................................................................................................................ 11

Figure 5. Diagram showing delta-deuterium composition and carbon-14 activity of water from selected 

observation wells along Sections B–B' and C–C', Mojave River ground-water basin, 

southern California ............................................................................................................................ 12

Figure 6. Diagrams showing delta-deuterium composition and carbon-14 activity of water from

selected observation wells along Sections D–D' and E–E', Mojave River ground-water 

basin, southern California .................................................................................................................. 13

Figure 7. Graph showing carbon-14 activities and tritium data in water from wells in the 

Mojave River and the Morongo ground-water basins, southern California, 1992–2000 .................. 15

Figure 7. Graph showing carbon-14 activities and tritium data in water from wells in the

Mojave River and the Morongo ground-water basins, southern California, 1992–2000 .................. 16

Figure 8. Flow chart showing interpretation of changes in chemistry, thermodynamics, mass transfer, 

and isotopic composition of water from wells along a flow path...................................................... 19

Figure 9. Graphs showing chemistry, mass transfer, and isotopic composition of water from 

selected wells along flow path 1 through the Surprise Spring basin, Morongo groundwater basin, southern California ........................................................................................................ 22

Figure 10. Graphs showing mass transfer and isotopic composition of water from selected wells along 

flow path 2 through the upper part of the Mojave River ground-water basin east of the 

Mojave River, southern California .................................................................................................... 25

Figure 11. Graphs showing mass transfer and isotopic composition of water from selected wells 

along flow path 3 through the upper part of the Mojave River ground-water basin 

west of the Mojave River, southern California.................................................................................. 30

Figure 12. Graphs showing comparison of uncorrected carbon-14 ages and interpreted carbon-14 

ages for ground water in the Mojave River and the Morongo ground-water basins, 

southern California ............................................................................................................................ 32
Tables v

TABLES

Table 1. Physical properties, chemical composition, isotopic composition, and saturation indexes, 

with respect to selected mineral phases, of water from selected wells along flow path 1

through the Surprise Springs subbasin, Morongo ground-water basin, southern California............. 21

Table 2. Measured and calculated delta carbon-13 composition, carbon-14 activity, and uncorrected

and interpreted carbon-14 ages for water from wells along selected flow paths through

the Mojave River and the Morongo ground-water basins, southern California ................................ 23

Table 3. Physical properties, chemical composition, isotopic composition, and saturation indexes, 

with respect to selected mineral phases, of water from selected wells along flow path 2

through the regional aquifer east of the Mojave River, southern California ..................................... 26

Table 4. Physical properties, chemical composition, isotopic composition, and saturation indexes, 

with respect to selected mineral phases, of water from selected wells along flow path 3 

through the regional aquifer west of the Mojave River, southern California .................................... 29
vi Conversion Factors , Vertical Datum, and Abbreviations

CONVERSION FACTORS, VERTICAL DATUM, AND ABBREVIATIONS

CONVERSION FACTORS 

Temperature in degrees Celsius (oC) may be converted to degrees Fahrenheit (oF) as follows:

oF=1.8 oC+32.

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) or 

micrograms per liter (μg/L). 

VERTICAL DATUM

Sea level: In this report, “sea level” refers to the National Geodetic Vertical Datum of 1929 (NGVD 

of 1929)—a geodetic datum derived from a general adjustment of the first-order level nets of both the 

United States and Canada, formerly called Sea Level Datum of 1929. 

ABBREVIATIONS

Ca calcium

Ca2+ calcium ion having a charge of +2

δ13C delta carbon-13

14C carbon-14

Cl chloride

Cl-1 chloride ion having a charge of -1

> greater than

HCO-1 bicarbonate ion having a charge of -1

Mg magnesium

Mg2+ magnesium ion having a charge of +2

Na sodium

Na+ sodium ion having a charge of +1

Multiply By To obtain

kilogram (kg) 2.205 pound avoirdupois

kilometer (km) 0.6214 mile 

square kilometer (km2) 247.1 acre

liter (L) 0.2642 gallon

meter (m) 3.281 foot 

meter per year (m/yr) 3.281 foot per year 

millimeter per year (mm/yr) 0.03937 inch per year 
Conversion Factors, Vertical Datum, and Abbreviations vii

NETPATH computer program for modeling net geochemical reactions along a flow path

NWIS National Water Information System

PDB Peedee Belemnite

pmc percent modern carbon

per mil parts per thousand

3H tritium

TU tritium units

ybp years before present

Organizations

MCAGCC U.S. Marine Corps facility at 29 Palms, California

USGS U.S. Geological Survey
Introduction 1

Movement and Age of Ground Water 

in the Western Part of the Mojave Desert, 

Southern California, USA

By John A. Izbicki and Robert L. Michel

ABSTRACT

Tritium and carbon-14 data in water from 

wells in the Mojave River and the Morongo groundwater basins in the western part of the Mojave 

Desert show recent recharge focused in the 

floodplain aquifer along the Mojave River. Older 

ground water was present in parts of the regional 

aquifer that surround and underlie the floodplain 

aquifer. Movement of water between the floodplain 

and the regional aquifers occurs near on the 

upgradient side of faults as water from the regional 

aquifer discharges to the floodplain aquifer and on 

the downgradient side of the faults where water from 

the floodplain aquifer recharges the regional aquifer. 

On the basis of carbon-14 ages, corrected for 

mineralogic reactions with aquifer materials, water 

from some wells was recharged more than 20,000 

years ago. Geochemical data show ground-water 

recharge has gradually decreased as the climate 

changed since that time.

INTRODUCTION

The Mojave River and the Morongo ground-water 

basins (referred to as the Mojave River and the Morongo 

basins, respectively) are in the western part of the Mojave 

Desert in southern California (fig. 1). The area is arid 

with cold winters and hot, dry summers. Population is 

growing rapidly and ground water is the only dependable 

source of water supply; however, ground-water recharge 

is small in comparison with the amount of water pumped. 

With increases in population and demand for water, water 

levels are declining (Mendez and Christensen, 1997), and 

there is increasing need to understand ground-water 

recharge and movement of water through the aquifers. 

Purpose and Scope

The purpose of this study was to evaluate the 

movement and age of ground water in the Mojave River 

and the Morongo ground-water basins in the western part 

of the Mojave Desert. In this paper, the radioactive 

isotopes of hydrogen (tritium) and carbon (carbon-14) 

are used to determine the age of water from wells, the 

timing of ground-water recharge, and the movement of 

water between aquifers. 

Background and Previous Studies

The combination of the radioactive isotopes of 

hydrogen (tritium) and carbon (carbon-14) is especially 

effective in arid environments for locating sources of 

recharge, determining the age (time since recharge) of 

ground water, and identifying geologic controls on the 

movement of ground water. 

Tritium

Tritium (3H) is a naturally occurring radioactive 

isotope of hydrogen that has a half-life of 12.43 years. In 

this study, tritium was measured in tritium units (TU); 

one tritium unit is equivalent to one tritium atom in 1018

atoms of hydrogen (Taylor and Roether, 1982). Because 

tritium is part of the water molecule, tritium is not 

affected by reactions other than radioactive decay; 

therefore, tritium is an excellent tracer of the movement 

of ground water recharged less than 50 years before 

present. 
2 Movement and Age of Ground Water in the Western Part of the Mojave Desert, Southern California, USA

R8W R1W R1E R9E

T

1

N

T

9

N

T

32

S

R41E R 45 E

Surprise

Spring

40

15

15

15

215

A A'

B B'

D–D'

C'

C

E E'

Troy

Lake

(Dry)

Lucerne

Lake

(Dry)

Coyote Lake

Harper (Dry)

Lake

(Dry)

El Mirage Lake

(Dry)

Deadman

Lake

(Dry)

Mesquite

Lake

(Dry)

Victorville

Barstow

Yucca

Valley

Mojave River

Afton Canyon

San Gabriel Mts

C

Pass

ajon

San Bernardino Mts

Helendal e Fault

San Andreas Fault

Water man

F ault

Surprise Spring Fault

Mojave River

ground-water

basin

Morongo

ground-water

basin

Mojave

Valley Camp

Cady Hinkley

Valley

Summit

Valley

Rock

Springs

Pipes

Wash

Sheep

Creek

Upper

Narrows

Lower

Narrows

11700' 11600'

34

30'

35

00'

C

A

L

I F O R N I A

STUDY

AREA

Los

Angeles

Pacific Ocean

Mojave

Desert

EXPLANATION

Dry lake

Selected major fault

Geologic section line

Areal extent of the

floodplain aquifer

D–D'

0 20

0 20

KILOMETERS

MILES

Figure 1. Location of study area, the Mojave River and the Morongo ground-water basins, the floodplain aquifer, selected faults, and geologic section 

lines.
Introduction 3

Tritium deposition at selected sites throughout 

the world has been measured by the International 

Atomic Energy Agency (1981) since the 1950s, and 

Michel (1989) estimated tritium deposition at 

unmeasured sites in the United States. The tritium 

concentration of precipitation in coastal California 

prior to 1952 was about 2 TU (International Atomic 

Energy Agency 1981; Michel, 1989) and, on the basis 

of Michel's (1989) data, tritium concentrations in 

precipitation in the southwestern United States 

increased with increasing distance from the coast. 

During 1952–62 about 800 kg of tritium was released 

to the atmosphere as a result of the atmospheric testing 

of nuclear weapons (Michel, 1976), and the tritium 

concentration of precipitation increased to about 700 

TU at a coastal site near Santa Maria, California. 

Tritium concentrations in precipitation at sites farther 

inland in the continental United States were higher 

(Michel, 1989). After the cessation of atmospheric 

testing of nuclear weapons in 1962, the tritium 

concentration of precipitation decreased and presentday (2002) tritium levels in precipitation are near pre1952 levels (Michel, 1989).

Begemann and Libby (1957) first demonstrated 

the use of tritium as a tracer of the movement of surface 

water, ground water, and oceanic circulation during the 

1950s. In arid areas, tritium is especially useful for 

identifying areas where ground-water recharge occurs 

under present-day climatic conditions (Froehlich and 

Yurtsever, 1995). In the Mojave Desert of the 

southwestern United States, tritium has been used to 

estimate movement of water through thick unsaturated 

zones (Phillips, 1994; Striegl and others, 1996; Izbicki 

and others, 2000) and to define the areal extent of 

present-day recharge in the saturated zone underlying 

the upper part of the Mojave River (Izbicki and others, 

1995). Most recently, tritium has been used in 

combination with its decay product helium-3 to 

produce more accurate estimates of ground-water ages 

and traveltimes in a wide range of hydrologic settings 

(Solomon and Cook, 1999).

Carbon-14 and Carbon-13

Carbon-14 (14C) is produced by interactions 

between cosmic rays and nitrogen gas in the earth's 

atmosphere and has a half-life of about 5,730 years 

(Mook, 1980). Carbon-14 data are expressed as percent 

modern carbon (pmc) by comparing 14C activities to 

the specific activity of National Bureau of Standards 

oxalic acid: 12.88 disintegrations per minute per gram 

of carbon in the year 1950 equals 100 pmc. Carbon-14 

production varies with solar activity and has not been 

constant during the last 24,000 years (Stuiver and 

others, 1998). In addition to natural sources, 14C also 

was produced by the atmospheric testing of nuclear 

weapons (Mook, 1980), and 14C activities may exceed 

100 pmc in areas where ground water contains tritium 

from nuclear weapons tests. 

Libby and others (1949) and Libby (1952) first 

discovered the ubiquitous distribution of natural 14C in 

the biosphere. Since the early 1960s, 14C present in 

ground water as dissolved inorganic carbon has been 

used in hydrologic studies to determine the time since 

recharge of older ground water (Vogel and Ehhart, 

1963). Carbon-14 activities are used to determine the 

age (time since recharge) of ground water on time 

scales ranging from recent to more than 20,000 years 

before present. Because 14C is not part of the water 

molecule, its activity and interpreted 14C ages may be 

affected by reactions between constituents dissolved in 

ground water and aquifer materials. 

Carbon-13 (13C), a naturally occurring stable 

isotope of carbon, is used in conjunction with chemical 

and mineralogic data to evaluate chemical reactions 

that occur within aquifers. These reactions may add 

carbon that does not contain 14C to the dissolved phase 

or remove carbon the may contain 14C from the 

dissolved phase. Carbon-13 data are expressed in delta 

notation (δ) as per mil (parts per thousand) differences 

relative the ratio of 13C to 12C in standard Peedee 

Belemnite (PDB) (Gonfiantini, 1978, 1984). 
4 Movement and Age of Ground Water in the Western Part of the Mojave Desert, Southern California, USA

Carbon-14 data have been used to evaluate 

ground-water ages in large aquifer systems throughout 

the arid southwestern United States. Winograd and 

Pearson (1976) used 14C data to estimate ground-water 

ages and identify interbasin flow in carbonate aquifers 

of southern Nevada. Phillips and others (1989a,b) used 

14C dating for ground water in the San Juan Basin of 

New Mexico. Application of 14C dating techniques to 

hydrologic studies has increased with the development 

of computer programs to assess rock-water interactions 

(Plummer and others, 1991) and advances in analytical 

techniques that have simplified sample collection. 

Robertson (1992), Kalin (1994), and Zhu and Murphy 

(2000) used 14C data to estimate ground-water ages, 

direction of ground-water flow, and aquifer hydraulic 

properties in several different basins representing a 

range of hydrogeologic settings in Arizona. Izbicki and 

others (1995) used 14C data to demonstrate that water 

from wells at the downgradient ends of long flow paths 

through alluvial aquifers in the Mojave Desert of 

southern California had great age and was recharged 

many thousands of years ago. 

Numerous researchers have recognized that older 

ground water in large aquifers reflects the 

environmental and climatic conditions at the time 

ground water was recharged rather than present-day 

conditions (Phillips and others, 1989a; Verhagen and 

others, 1991; Zhu and others, 1998). Ground water in 

large aquifers in arid areas has become increasingly 

important as an archive of paleoclimatic data because 

most of the water in these aquifers may have been 

recharged during pluvial periods when the climate was 

significantly different from present-day conditions. 

This concept is somewhat controversial in areas such as 

the regional carbonate aquifer underlying southern 

Nevada where Davisson and others (1999) suggested 

that low 14C activities could be explained by accretion 

of present-day recharge having low 14C activity as a 

result of reactions with carbonate aquifer material.

Sample Collection and Analyses

Tritium was measured by liquid scintillation 

(Thatcher and others, 1977) with electrolytic 

enrichment (Ostlund and Warner, 1962) on 1-L 

unfiltered samples collected in glass containers. 

Sample bottles were bottom-filled and allowed to 

overflow several times the bottle volume. The 2-sigma 

detection limit for tritium ranged from 0.2 to 1.2 TU 

and most samples had a detection limit of 0.4 TU. The 

detection limit was a function of the statistics 

associated with the liquid scintilation counter. In 

general, the longer the counting time the better the 

precision and the lower the detection limit. 

Prior to 1994, 14C samples were collected in a 

96-L nitrogen-filled stainless-steel vessel. The pH of 

unfiltered sample water was adjusted to pH>11 using 

carbon-dioxide-free sodium hydroxide, and 

bicarbonate was precipitated as strontium carbonate by 

supersaturating the sample with strontium chloride. 

The strontium carbonate precipitate was allowed to 

settle into a 1-L polyetheylene bottle at the bottom of 

the stainless-steel vessel. The 1-L bottle was shipped to 

a laboratory where 14C analyses were done by 

scintillation spectrophotmetry (Thatcher and others, 

1977). After 1994, 14C samples were filtered and 

collected in 1-L glass bottles, in the same manner as 

tritium, and were analyzed for 14C by accelerator mass 

spectrometry (Beukens, 1992). Carbon-13 analyses 

were done on filtered water samples by mass 

spectrometry (Thatcher and others, 1977). 

Isotopic and chemical data are available from the 

U.S. Geological Survey's National Water Information 

System (NWIS) data base. Lithologic data, geophysical 

data, well construction, water-level measurements, and 

results of samples collected from observation wells 

installed as part of this study were published by 

Huff and others (2003).

Acknowledgments

This study was funded cooperatively by the 

Mojave Water Agency of Apple Valley, California, and 

the U.S. Geological Survey (USGS); additional 

funding was provided by the U.S. Marine Corps 

MCAGCC at 29 Palms, California, and by the USGS's 

Southwestern Ground-Water Resources Initiative. The 

authors thank James Ball and James Borchers for their 

assistance with the preparation of this paper.
Hydrogeology 5

HYDROGEOLOGY

The study area and surrounding uplands includes 

about 13,000 km2 of the Mojave Desert east of the San 

Gabriel and the San Bernardino Mountains east of Los 

Angeles, California. With the exception of the higher 

altitudes in the San Gabriel and the San Bernardino 

Mountains, the climate of the study area is 

characterized by low precipitation and low humidity. 

The temperature commonly excees 40oC during the 

summer and frequently falls below 0oC during the 

winter. In the northern and western parts of the study 

area most precipitation falls during the winter rainy 

season (November–March); in the southeastern part of 

the Morongo basin more precipitation falls during the 

summer months (July–September) than during the 

winter rainy season (Freidman and others, 1992). 

Although average annual precipitation in the higher 

altitudes of the mountains may exceed 1 m/yr; average 

annual precipitation in most of the study area is less 

than 150 mm/yr. 

With the exception of small streams in the San 

Gabriel and the San Bernardino Mountains and short 

reaches of the Mojave River where faults and other 

barriers to ground-water flow cause ground water to 

discharge at land surface, there are no perennial 

streams in the study area. During occasional winter 

stormflows, the Mojave River flows east past Barstow 

into the Mojave Valley (fig. 1), and if flows are large 

enough through Afton Canyon out of the study area 

(Lines, 1996). Other large streams, such as Pipes Wash 

in the Morongo basin, flow for considerable distances 

during brief stormflows. A brief discussion of groundwater occurrence and movement is provided in this 

paper. Additional information on ground water 

occurrence and movement is given by Thompson 

(1929), Hardt (1971), and Stamos and others (2001).

Mojave River Ground-Water Basin

The Mojave River ground-water basin contains 

an unconsolidated alluvial aquifer along the Mojave 

River known as the floodplain aquifer. The floodplain 

aquifer is highly permeable and composed of sand and 

gravel weathered from granitic rocks in the San Gabriel 

and the San Bernardino Mountains. The floodplain 

aquifer is surrounded and underlain by unconsolidated 

alluvial deposits that constitute an aquifer known 

locally as the regional aquifer. The regional aquifer 

includes interconnected alluvial basins that drain 

toward the Mojave River. The regional aquifer also 

includes topographically closed basins that drain 

toward El Mirage, Harper, and Coyote Lakes (dry). In 

the geologic past, flow in the Mojave River may have 

reached Harper Lake, maintaining surface water in the 

lake (Meek, 1999). The regional aquifer is less 

permeable than the floodplain aquifer, except where 

highly-permeable deposits from the ancestral Mojave 

River are present. Lacustrine deposits are present 

throughout the area near present-day playas and in 

areas, such as near Victorville or in the Mojave Valley, 

where large lakes were present during the Pleistocene 

(Cox and Hillhouse, 2001). In some areas, especially 

near the front of the San Bernardino and the San 

Gabriel Mountains, ground-water pumping is limited 

by the depth to water which commonly approaches 

100 m and may exceed 300 m. 

Under present-day climatic conditions, recharge 

to the regional aquifer is small compared to recharge 

from infiltration of surface flow from the Mojave River 

(Lines, 1996; Stamos and others, 2001). The recharge 

that does occur results primarily as infiltration from 

small streams near the flanks of the San Bernardino 

and the San Gabriel Mountains that flow as a result of 

winter stormflows and snowmelt runoff (Hardt, 1971; 

Lines, 1996; Izbicki and others, 1998, 2000; Stamos 

and others, 2001). Smaller amounts of recharge also 

may occur as infiltration of runoff from streams that 

drain the mountains in the Mojave Desert. As in most 

of the desert southwest, recharge from the direct 

infiltration of precipitation typically does not occur in 

the study area and large accumulations of chloride and 

other soluble salts are present near the top of the 

unsaturated zone (Izbicki and others, 1998, 2000). 
6 Movement and Age of Ground Water in the Western Part of the Mojave Desert, Southern California, USA

Under predevelopment conditions, ground-water 

movement in much of the Mojave River ground-water 

basin was from sources of recharge to the regional 

aquifer near the front of the San Gabriel and the San 

Bernardino Mountains to the floodplain aquifer where 

it discharged as streamflow to the Mojave River or as 

evapotranspiration by phreatophytes (Hardt, 1971; 

Stamos and others, 2001). Smaller amounts of 

discharge occurred as ground-water movement toward 

dry lakes where water discharged by evaporation 

(Stamos and others, 2001). The area is highly faulted 

and faults can act as barriers to ground-water flow in 

both the floodplain and the regional aquifers, often 

causing ground water to discharge at land surface 

(Thompson, 1929; Hardt, 1971; Mendez and 

Christensen, 1997; Stamos and others, 2001). As a 

result of ground-water development, pumping is the 

largest source of ground-water discharge in the study 

area under present-day conditions (Stamos and others, 

2001) and in some areas water levels are declining at 

rates exceeding 0.3 m/yr (Mendez and Christensen, 

1997).

Morongo Ground-Water Basin

The Morongo ground-water basin includes a 

number of small alluvial subbasins that maintain 

separate ground-water flow systems typically 

terminating in dry lakes scattered throughout the area 

(Lewis, 1972; Mendez and Christensen, 1997). These 

alluvial subbasins are separated by faults and bedrock 

outcrops. Under predevelopment conditions perennial 

springs, such as Surprise Spring, existed where ground 

water discharged at land surface as a result of faults 

acting as barriers to flow (Londquist and Martin, 1991). 

Perennial springs also were present near the margins of 

some dry lakes where water discharged by evaporation. 

Ground water near the dry lakes in the Morongo (and 

the Mojave) basins is typically highly saline. 

Unconsolidated deposits in the Morongo basin 

are similar to those found in the regional aquifer 

underlying the Mojave River basin and are weathered 

from the surrounding mountains composed of similar 

crystalline bedrock, with the exception of deposits 

weathered from limestone present along the eastern 

edge of the San Bernardino Mountains near Lucerne 

Valley (and adjacent parts of the Mojave River groundwater basin). In some areas, alluvial aquifers in the 

Morongo basin have been divided into upper and lower 

aquifer systems having different water levels and 

distinct hydrologic properties (Londquist and Martin, 

1991). 

TRITIUM

In this paper, water containing tritium is 

interpreted as water recharged after 1952, and water not 

containing measurable tritium is interpreted as water 

recharged before 1952. In areas where the tritium 

concentrations along a flow path are known, it is 

possible to identify the peak tritium concentrations that 

correspond to ground water recharged during 1962 

when the atmospheric deposition of tritium from 

nuclear weapons testing was greatest (Izbicki and 

others, 1995). In some settings, low concentrations of 

tritium may be difficult to interpret without 

understanding the local geologic setting or the history 

of well installation and development. For example, 

mixing of younger water containing measurable tritium 

and older water not containing tritium may occur 

within long-screened production wells. Incomplete 

well development and removal of drilling fluids may 

complicate interpretation of low concentrations of 

tritium collected from short-screened observation 

wells. 

Tritium was detected in 46 percent of water 

samples from 234 wells in the Mojave River and the 

Morongo ground-water basins at values as high as 

14.9 TU. Concentrations of tritium near the 2-sigma 

detection limit may accurately reflect the presence of 

post-1952 ground-water recharge or may result from 

effects related to well construction or development. 

Water from 25 observation wells installed as part of 

this study was resampled to confirm the presence or 

absence of low concentrations of tritium.

Most wells having measurable tritium were in 

the floodplain aquifer along the Mojave River upstream 

from Mojave Valley. In the Mojave Valley, tritium is 

present only in water from wells adjacent to the river, 

and much of the floodplain aquifer in this area contains 

water that was recharged prior to 1952. Tritium also is 

present in water from wells in alluvial fans and canyons 

along the front of the San Gabriel and the San 

Bernardino Mountains in the Mojave River and the 

Morongo basins (fig. 2) where recharge occurs as 

infiltration of flow in streams that drain the mountains 

and along alluvial fans at the base of the mountains.
Tritium 7

R9W R1W R1E R9E

T

1

N

T

9

N

T

32

S

R41E R 45 E

Harper

Lake

(Dry)

Helendal e Fault

Water man

F ault

Mojave

Valley Hinkley

Valley

Morongo

ground-water

basin

Selected faults

Wells—

Mojave River

ground-water

basin

Tritium activity

Dry lake

Multiple-well site

Sampled for tritium

EXPLANATION

Presence or absence of tritium interpreted

from carbon-14 activity

Approximate boundary

of floodplain aquifer

Water containing tritium Mojave River

11700' 11600'

34

30'

35

00'

Victorville

Barstow

Yucca

Valley

San Gabriel Mts

San Bernardino Mts

Sheep

Creek

Pipes

Wash

0 20

0 20

KILOMETERS

MILES

Figure 2. Tritium in water from wells in the Mojave River and the Morongo ground-water basins, western Mojave Desert, southern California, 

1992–2000.
8 Movement and Age of Ground Water in the Western Part of the Mojave Desert, Southern California, USA

Tritium was detected in water from some 

production wells in the regional aquifer. In the upper 

part of the basin near Victorville, extensive pumping 

from the regional aquifer has caused water levels to 

decline; allowing the movement of water from the 

floodplain aquifer into the regional aquifer. Farther 

downstream near Hinkley Valley, tritium data suggest 

rapid movement of ground water from the floodplain 

aquifer into the regional aquifer and toward discharge 

areas near Harper Lake. In addition, tritium also was 

detected in observation wells in the regional aquifer 

and underlying deposits downgradient from the 

Helendale and the Waterman Faults. 

CARBON-14 AND CARBON-13

Inorganic carbon-14 activities for water from 

167 wells in the Mojave River and Morongo basins 

ranged from 117 to 0.5 percent modern carbon (pmc). 

The δ13C compositions of water these wells ranged 

from 0.1 to −19.2 per mil. In general, δ13C values 

increased (became less negative) with decreasing 14C 

activity. Carbon-14 and δ13C data were used to 

estimate the areal distribution of ground-water ages 

(time since recharge) in areas where water from wells 

did not contain tritium. These data also were used to 

evaluate the movement of water through aquifers, the 

hydraulic connection between aquifers, and the effect 

of faults on ground-water movement along selected 

geologic sections. Corrections to 14C activity and 

ground-water ages for reactions that may add or 

remove carbon from the ground water were made for 

water from wells along three flow paths that represent 

different geochemical settings in the study area. These 

corrections were made on the basis of changes in δ13C 

compositions and changes in the chemical composition 

of water from wells along selected flow paths through 

the aquifers using the computer program NETPATH 

(Plummer and others, 1991). 

Areal Distribution of Carbon-14 Activities 

The highest 14C activities, exceeding 90 pmc, 

were in water from wells in the floodplain aquifer 

along the Mojave River and in other areas where 

tritium and recent recharge are present (fig. 3). Water 

from wells exceeded 100 pmc where recently 

recharged water containing tritium is present and the 

14C activity of ground water has been increased by the 

atmospheric testing of nuclear weapons. Carbon-14 

activities decrease downgradient from areas of recent 

ground-water recharge and are between 50 and 90 pmc 

in parts of the regional aquifer within the Mojave River 

basin (1) near the front of the San Gabriel and San 

Bernardino Mountains where recharge occurs as 

infiltration of flow in streams that drain the mountains 

and along the alluvial fans at the base of the mountains, 

(2) near Cajon Pass where recharge occurs as 

infiltration from intermittent streams and as groundwater movement from Summit Valley, and (3) in 

Hinkley and Mojave Valleys where ground water is 

recharged by infiltration of surface flows from the 

Mojave River. Carbon-14 activities also are between 50 

and 90 pmc in area near Yucca Valley, along Pipes 

Wash, and downgradient from streams that drain the 

San Bernardino Mountains near Lucerne Valley

(fig. 3). Carbon-14 activities less than 30 pmc are 

present throughout much of the Mojave River basin 

away from the front of the San Gabriel and the San 

Bernardino Mountains and in large areas of the 

Morongo basin.

Assuming only radioactive decay, 100 pmc in the 

recharging ground water, and neglecting geochemical 

reactions that occur between ground water and aquifer 

materials, ground water having 90 pmc would have 

recharged 370 years before present, and ground water 

having 50 pmc would have been recharged 5,730 years 

before present. (These ages are referred to as 

“uncorrected ages”; ground-water ages corrected for 

the initial 14C activity of recharging ground water and 

geochemical reactions that occur between ground water 

and aquifer materials will be presented for selected 

wells later in this paper.) Much of the ground water in 

the regional aquifer in the Mojave River and the 

Morongo basins has 14C activities less than 30 pmc and 

an uncorrected ground-water age greater than about 

10,000 years before present. 

The lowest 14C activities, less than 2 pmc, 

having uncorrected ground-water ages greater than 

30,000 years before present, are in water from deep 

wells at the downgradient ends of long flow paths 

through the regional aquifer in the Mojave River basin. 

This water often underlies younger ground water that 

has higher 14C activity and is not apparent in figure 3. 
Carbon-14 and Carbon-13 9

R9W R1W R1E R9E

T

1

N

T

9

N

T

32

S

R41E R 45 E

Flow path 3

3A2 1C2

22E2

4D4

33J1

Flow path 2

9Q1

27E1

1R1

26J1

19G1

16A1

19N1

29F1

29R1

3A1

35P2

Morongo

ground-water

basin

Mojave River

ground-water

basin

11700' 11600'

34

30'

35

00'

Victorville

Barstow

Yucca

Valley

Selected faults

Direction of groundwater flow

Wells—

Carbon-14 activity, in percent modern

carbon (pmc). Dashed where uncertain

Multiple-well site

Sampled for carbon-14

Carbon-14 inferred from tritium data

EXPLANATION

Approximate boundary

of floodplain aquifer

Mojave River Less than 30

30 to 50

50 to 90

Greater than 90

Pipes

Wash

Flow path 2 Flow path 1

Flow path 3

Flow path 1

Sheep

Creek

San Gabriel Mts

San Bernardino Mts

0 20

0 20

KILOMETERS

MILES

Dry lake

C

Pass

ajon

Mojave

Valley Hinkley

Valley

Lucerne

Valley

Summit

Valley

Figure 3. Carbon-14 activity in water from wells in the Mojave River and the Morongo ground-water basins, western Mojave Desert, southern California, 

1992–2000.
10 Movement and Age of Ground Water in the Western Part of the Mojave Desert, Southern California, USA

Carbon-14 Activities Along Selected Geologic 

Sections in the Mojave River Basin

Changes in 14C activity with depth in water from 

selected observation wells were evaluated along five 

geologic sections in the Mojave River basin to 

determine the movement of water between the regional 

and the floodplain aquifers. Sections A–A', B–B' and 

C–C' (figs. 4 and 5) are perpendicular to the Mojave 

River and through different reaches of the floodplain 

and the regional aquifers. Each section represents 

hydrologic conditions in the basin (1) upstream from 

Victorville (Section A–A'), (2) downstream from 

Victorville in an area known locally as the “transition 

zone” (Section B–B'); and (3) farther downstream in 

the Mojave Valley (Section C–C'). Sections D–D' and 

E–E' (fig. 6) are parallel to the Mojave River and cross 

the Helendale and the Waterman Faults, respectively. 

These sections represent hydrologic conditions 

upgradient and downgradient from the faults, which are 

barriers to ground-water flow. 

Carbon-14 activity was not measured on all wells 

sampled for tritium, but where measured, 14C activity 

in the floodplain aquifer commonly exceeded 100 pmc 

as a result of the atmospheric testing of nuclear 

weapons.

Sections A–A', B–B', and C–C′

Section A–A' is located in the upper part of the 

Mojave River basin near Victorville. The 14C activities 

of water from sampled observation wells in the 

regional aquifer along section A–A' ranged from 95 to 

0.5 pmc (fig. 4). Although the 14C activity of water 

from sampled observation wells decreased with depth 

to low values across the entire section, there was not a 

sharp contrast in ground-water age between the 

floodplain and the regional aquifers. Water from wells 

in the floodplain aquifer contains tritium and was 

recently recharged. Although ground water discharged 

to the floodplain aquifer along this reach under predevelopment conditions (Hardt, 1971; Stamos and 

others 2001), some water may have moved from the 

floodplain aquifer into the regional aquifer as a result 

of ground-water pumping. 

Section B–B' is downstream from Victorville in 

an area known locally as the “Transition Zone.” The 

14C activities of water from sampled observation wells 

in the regional aquifer along section B–B' ranges from 

87 to 2.7 pmc (fig. 5). The lowest 14C activities were in 

deep wells beneath the floodplain aquifer where 

ground water from the regional aquifer discharged to 

the overlying floodplain aquifer under predevelopment 

conditions. Similar to section A–A' water from wells in 

the floodplain aquifer contains tritium and was recently 

recharged. The contrast in ground-water age is much 

greater along the western part of section B–B' than 

along section A–A', reflecting predevelopment ground 

water discharge from the regional aquifer to the 

floodplain aquifer along this section. Carbon-14 

activities to the east of section B–B' are relatively high 

and ground water in this area does not have great age 

suggesting movement of water between the aquifers in 

this area.

Section C–C' is located farthest downstream in 

the Mojave Valley. The floodplain aquifer in this area is 

larger and ground-water flow paths through this area 

are longer than along either sections A–A' or B–B'. The 

quantity of recharge to the regional aquifer from the 

surrounding mountains is small. Large amounts of 

ground-water discharge from the regional aquifer to the 

floodplain aquifer did not occur in this area under 

predevelopment conditions (Hardt, 1971; Stamos and 

others, 2002) and water infiltrated from the Mojave 

River is present in both the floodplain and the regional 

aquifers along this section. The 14C activity of water 

from sampled wells along section C–C' ranged from 

1 to 34 pmc (fig.5). Along this section and throughout 

most of the Mojave Valley, recent water is present only 

in the part of the floodplain aquifer adjacent to the 

Mojave River and carbon-14 activities are low 

throughout the remainder of the floodplain and the 

regional aquifers. The lowest 14C activities were in 

wells completed in the underlying consolidated rocks. 
Carbon-14 and Carbon-13 11

Qa

QTu

EXPLANATION

QToa

Mojave River alluvium

Water table

Floodplain aquifer

Well and number—

Carbon-14 activity,

Regional aquifer

Undifferentiated alluvium

Older alluvium of ancestral

Mojave River

Igneous, metamorphic,

or volcanic rocks

Fault—

Dashed where projected.

Sequence numbers are

from deepest to shallowest.

White box represents

screened interval.

Recent water containing tritium

Older water not containing tritium

Percent modern carbon

No data

Dashed where

uncertain.

delta Deuterium, in per mil

Less than –80

–70 to –80

Greater than –70

4N/4W-3A2–5

Vertical scale greatly exaggerated

WEST

A

1,100

1,000

900

800

700

600

500

400

ALTITUDE, IN METERS ABOVE SEA LEVEL

0 5 MILES

5 KILOMETERS

A'

Regional aquifer

Floodplain

aquifer

0

EAST

5N/6W-22E1–3

4N/3W-12A1–3

5N/3W-27E3–6

4N/4W-3A2–5

4N/4W-1C2–5

4N/5W–1M1

QToa QToa

QTu QTu

QTu QTu

QTu

Mojave River

Qa

11

11

R

NR

R

NR

—

—

11

—

95

83

R

R

0.5

36

41

4

— —— —

—

—

—

1.2

Figure 4. Delta-deuterium composition and carbon-14 activity of water from selected observation wells along Section A–A', Mojave River ground-water 

basin, southern California. See figure 1 for location of section. (Modified from Stamos and others, 2001; delta deuterium data from Izbicki, 2004)
12 Movement and Age of Ground Water in the Western Part of the Mojave Desert, Southern California, USA

11

11

R

NR

R

NR

— —

0

0

2 MILES

2 KILOMETERS

C C'

NORTH SOUTH Mojave River

ALTITUDE, IN METERS ABOVE SEA LEVEL

350

400

500

575

9N/3E-22R4–7

10N/3E-27J1–5

QToa

QTu

QTu

Qa

0 Vertical scale greatly exaggerated

0

2 MILES

2 KILOMETERS

ALTITUDE, IN METERS ABOVE SEA LEVEL

B B'

WEST EAST

600

700

800

850

7N/5W-23R1–3

7N/5W-24R6–8

7N/5W-24R6–8

7N/4W-19Q5–7

QTu

QTu

Qa

Mojave River

QToa

11

34

3.1

R

NR

NR

45

3.3

2.7

2.7

4.3

R

R 87

57

19 22

4

1

Qa

QTu

EXPLANATION

QToa

Mojave River alluvium

Water table

Floodplain aquifer

Well and number—

Carbon-14 activity,

Regional aquifer

Undifferentiated alluvium

Older alluvium of ancestral

Mojave River

Igneous, metamorphic,

or volcanic rocks

Fault—

Dashed where projected.

Sequence numbers are

from deepest to shallowest.

White box represents

screened interval.

Recent water containing tritium

Older water not containing tritium

Percent modern carbon

No data

Dashed where

uncertain.

delta Deuterium, in per mil

Less than –80

–70 to –80

Greater than –70

Figure 5. Delta-deuterium composition and carbon-14 activity of water from selected observation wells along Sections B–B' and C–C', Mojave River 

ground-water basin, southern California. See figure 4 for explanation and figure 1 for location of sections. (Modified from Stamos and others, 2001; delta 

deuterium data from Izbicki, 2004.)
Carbon-14 and Carbon-13 13

11

11

R

NR R

NR

—

—

Qa

QTu

EXPLANATION

QToa

Mojave River alluvium

Water table

Floodplain aquifer

Well and number—

Carbon-14 activity,

Regional aquifer

Undifferentiated alluvium

Older alluvium of ancestral

Mojave River

Igneous, metamorphic,

or volcanic rocks

Fault—

Dashed where projected.

Sequence numbers are

from deepest to shallowest.

White box represents

screened interval.

Recent water containing tritium

Older water not containing tritium

Percent modern carbon

No data

Dashed where

uncertain.

delta Deuterium, in per mil

Less than –80

–70 to –80

Greater than –70 8N/4W-29E3–6 8N/4W-20Q7–11 8N/4W-21M1–4

Qa

D D'

SOUTHWEST NORTHEAST

QTu

QTu

ALTITUDE, IN METERS ABOVE SEA LEVEL

575

600

700

750

0

0

2 MILES

2 KILOMETERS

53

—

—

58

28

108

116

27

69

NR

—

—

32

Helendale Fault

FAULT B

FAULT C

FAULT A

FAULT E

FAULT D

? ? ??

?

WATERMAN FAULT 400

500

600

650

ALTITUDE, IN METERS ABOVE SEA LEVEL

WEST

5 MILES

0 5 KILOMETERS

0 Vertical scale

greatly exaggerated

QTu

QToa

Qa

Qa

9N/1W-10J12–15

9N/1W-11K12–15

9N/1W-12N4–7

9N/1E-16F1–4

9N/1E-16F1-4

9N/1E-10Q2–4

9N/1E-10R1

Bend in section

EAST

E E'

53

R

R

R

--

--

104

--

95

95

92

108

19 12

4

11.5

R

R

R

R

Figure 6. Delta-deuterium composition and carbon-14 activity of water from selected observation wells along Sections D–D' and E–E', Mojave River 

ground-water basin, southern California. See figure 4 for explanation and figure 1 for location of sections; per mil, parts per thousand. (Section D–D'

modified from Stamos and others, 2001. Section E–E' modified from Densmore and others, 1997; delta deuterium data from Izbicki, 2004.)
14 Movement and Age of Ground Water in the Western Part of the Mojave Desert, Southern California, USA

Sections D–D' and E–E'

Ground-water discharge supported phreatophyte 

growth under predevelopment conditions at the 

Helendale Fault near section D–D' (Thompson, 1929). 

The 14C activity of water sampled from wells along 

section D–D' ranged from 27 to 116 pmc (fig. 6). 

Carbon-14 activity in water from wells on the 

downgradient side of the fault is higher than 14C 

activity from deep wells on the upgradient side. 14C 

activity data are consistent with the downward 

movement of more recently recharged water from the 

floodplain aquifer into the regional aquifer and 

underlying consolidated rocks on the downgradient 

side of the Helendale Fault. The lowest 14C activities 

are in water from deep wells in consolidated rocks on 

the upgradient side of the Helendale Fault. This deep 

ground water is slightly saline, having dissolved-solids 

concentrations as high as 3,190 mg/L (Huff and others, 

2003).

Section E–E' crosses the Waterman Fault 

downstream from Barstow. The Waterman Fault is a 

barrier to flow in both the floodplain and the regional 

aquifers, and large changes in the water-table surface 

occur near the fault. The subsurface geology near the 

Waterman Fault is complex and the fault in this area is 

actually composed of five separate faults mapped as 

faults A through E (Densmore and others, 1997). The 

14C activity of sampled wells in the regional aquifer 

and underlying deposits along section E–E' ranged 

from 4 to 108 pmc (fig. 6). The higher 14C activities are 

in water from sampled wells in the regional aquifer on 

the downgradient side of the fault. The floodplain 

aquifer downgradient from the Waterman Fault is the 

recharge area of the Mojave Valley. Lower 14C 

activities in deep wells on the upgradient side of the 

Waterman Fault reflect discharge of deeper, older 

ground water to the floodplain aquifer in this area. 

Ground water from deeper wells upgradient from the 

Waterman Fault is similar to water from deep wells 

upgradient from the Helendale Fault in that it is slightly 

saline, having dissolved-solids concentrations as high 

as 2,300 mg/L.

Interpretation of Ground-Water Age Along 

Selected Flow Paths

Carbon-14 activities were used to estimate 

ground-water ages along selected flow paths in the 

Mojave River and the Morongo ground-water basins. 

Interpretation of 14C activities and calculation of 

ground-water ages requires estimates of the initial 14C 

activity of ground water after recharge and an 

evaluation of the geochemical reactions that have 

occurred within an aquifer. These reactions may 

remove inorganic carbon from the dissolved phase or 

add inorganic carbon from aquifer materials that may 

or may not contain 14C.

The effect of different geologic settings and 

geochemical reactions on 14C activities and interpreted 

ages was evaluated along three flow paths (fig. 3) 

developed on the basis of water-level-contour data. 

Flow path 1, located in the Surprise Springs area of the 

Morongo basin, represents ground-water evolution in a 

simple geochemical setting dominated by silicate 

weathering and influenced by recent climatic change. 

Flow path 2, located in the regional aquifer in the upper 

part of the Mojave River basin east of the Mojave 

River, represents a more complex geochemical setting 

where ground water has reacted with soluble carbonate 

minerals prior to and after recharge. Flow path 3, 

located in the regional aquifer in the upper part of the 

Mojave River basin west of the Mojave River, 

represents ground-water reactions with aquifer 

minerals in deep aquifers over long periods of time 

(fig. 3).
Carbon-14 and Carbon-13 15

Initial Carbon-14 Activity 

Neglecting geochemical reactions that occur 

within an aquifer, the age (time since recharge) of 

ground water can be estimated from the equation for 

radioactive decay according to the following:

(1)

where

The 14C activity of ground water at the time of 

recharge (A0) is rarely equal to 100 pmc because of 

reactions that occur between infiltrating water, soil 

gases (primarily carbon dioxide), and carbonate 

minerals in the unsaturated zone. It is not possible to 

measure this value directly by sampling at the watertable surface in ground-water recharge areas because 

atmospheric testing of nuclear weapons increased the 

14C activity of ground water recharged since 1952 to 

greater than natural, pre-bomb, values. 

A number of analytical approaches have been 

developed to calculate ground-water age that rely on 

estimates of the 14C activity of ground water at the time 

of recharge. Approaches developed by Vogel and 

Ehhart (1963), Tamers (1967), and Ingerson and 

Pearson (1964) assume equilibrium of recharging water 

with the atmosphere or soil gases, and account for the 

resulting dissolution of carbonate minerals within the 

unsaturated zone. These models are suitable for use in 

areas receiving large amounts of recharge where the 

unsaturated zone has been well leached and contains 

only small amounts of soluble carbonate minerals. 

Approaches developed by Fontes and Garnier (1979) 

and Mook (1980) extend these models to account for 

isotopic fractionation and isotopic exchange between 

ground water and carbonate minerals. These models 

are suitable for use in areas where the unsaturated zone 

has not been leached of soluble carbonate minerals, 

either because of their local abundance or because 

recharge amounts are small. Kalin (2000) and Zhu and 

Murphy (2000) summarized the use and limitations of 

these approaches to determine the initial 14C activity of 

ground water.

Although widely used, these analytical 

approaches are applicable for the specific, often 

simplified, geochemical settings for which they were 

developed and may not reflect geochemical conditions 

in an other study areas (Zhu and Murphy, 2000). To 

address this problem, the initial 14C activity of ground 

water in the study area was estimated through the 

collection of a many of ground-water samples in, and 

downgradient from, recharge areas. When using this 

approach, the greatest 14C activity of water from wells 

that no longer contains tritium is selected as the initial 

14C activity, A0, of ground-water recharge. If sufficient 

data are available, this approach provides a better 

estimate of the initial carbon-14 activity of ground 

water prior to nuclear weapons testing than do 

analytical estimates because it reflects conditions 

within the study area. The approach does not account 

for radioactive decay that occurred since 1952; 

however, given the half-life of 14C, the decrease in 14C 

activity resulting from radioactive decay since 1952 is 

small and can be neglected.

Carbon-14 data collected in the Mojave River 

and the Morongo basins are shown in rank order, 

expressed as percent greater than or equal to, from 

largest to smallest in figure 7. Carbon-14 activities 

exceeded 84 pmc in water samples from all but one 

well containing tritium, suggesting that this value may 

be an appropriate estimate of the initial carbon activity, 

A0, in parts of the study area where data are not 

available. This value agrees with results of analytical 

models developed by Vogel and Ehhart (1963), which 

predict the 14C activity of recharge water in wellleached, carbonate-poor settings should be about 

85±3 pmc. 

t is the age (time since recharge) of the 

ground water,

1/2 is the isotope half-life; for 14C the half-life 

is 5,730 years,

A0 is the 14C activity of the water sample at the 

time of recharge, and

Ameas is the measured 14C activity of the water 

sample.

t

λ1 2⁄

ln2 ---------- A0Ameas

= ln -------------
16 Movement and Age of Ground Water in the Western Part of the Mojave Desert, Southern California, USA

0 20 40 60 80 100

PERCENT GREATER THAN OR EQUAL TO

CARBON-14, IN PERCENT MODERN CARBON

Carbon-14 data

Wells having measurable tritium

A0 = 84 pmc

A0 = 63 pmc

Mixed water from wells

downgradient from the

Helendale and the Waterman Faults

Well-leached, carbonate-poor alluvium

Poorly-leached, carbonate-rich alluvium

0

20

40

60

80

100

120

Artificially

recharged

water

Figure 7. Carbon-14 activities and tritium data in water from wells in the Mojave River and the Morongo ground-water basins, southern California, 

1992–2000.
Carbon-14 and Carbon-13 17

Water from wells in ground-water recharge areas 

along the flanks of the San Bernardino Mountains in 

the Mojave River and the Morongo basins near Lucerne 

Valley that contains tritium has 14C activities between 

71 and 63 pmc. No large streams drain the mountains 

in this area, and much of the recharge occurs through 

alluvial fan deposits composed of detritus weathered 

from carbonate rocks in the surrounding mountains. 

Carbonate minerals are not likely to have been 

completely leached from these deposits. Lower 14C 

activities probably occur in ground water in this area as 

a result of dissolution and isotopic exchange with 

carbonate minerals. An initial 14C activity of 63 pmc is 

consistent with 14C activities predicted for recharge 

water in poorly-leached unsaturated zones using 

analytical models developed by Fontes and Garnier 

(1979) or Mook (1980).

Ground water containing measurable tritium but 

low 14C activity occurs in several other settings in the 

study area as a result of other processes. For example, 

water from well 9N/1W-12N5, located on the 

downgradient side of the Waterman Fault (fault C) 

(fig. 6), contains low levels of tritium and has a 

comparatively low carbon-14 activity of 53 pmc. Water 

from this well is a mixture of water from the floodplain 

and the regional aquifers, each containing different 

tritium concentrations and carbon-14 activities. In 

another setting, water from a short-screened watertable observation well beneath an artificial-recharge 

pond in Lucerne Valley in the Morongo basin also 

contains measurable tritium but has low 14C activity. 

Carbon-14 activity is interpreted to have decreased as a 

result of dissolution of carbon containing minerals that 

do not contain 14C and isotopic exchange with carbon 

in these minerals as recharge water infiltrated through 

the thick, carbonate-rich unsaturated zone beneath the 

pond. Similar processes also occur as a result of natural 

recharge through thick unsaturated zones beneath 

intermittent streams. Unsaturated zones beneath some 

stream reaches are often poorly leached and contain 

water near saturation with highly soluble calcium and 

sodium-carbonate minerals (Izbicki and others, 2000). 

Carbon-14 activity in a short-screened water-table well 

beneath a wash near Cajon Pass was 32 pmc (Izbicki 

and others, 2000). Small amounts of recharge occur at 

this site under present-day climatic conditions and the 

water is younger than 14C-activity data suggest. 

However, the water did not contain tritium because at 

least 250 years are required for water to infiltrate to the 

water table and recharge the aquifer 197 m below land 

surface (Izbicki and others, 2002).

Geochemical Reactions

Geochemical reactions that affect the age (time 

since recharge) of ground water can be incorporated 

into the equation for radioactive decay according to the 

following:

(2)

where Σ Areact is change in 14C activity that has 

occurred as a result of geochemical reactions between 

ground water and aquifer material since ground water 

was recharged. All other parameters have the same 

meaning as in equation 1.

Geochemical reactions that occur between 

ground water and aquifer material are evaluated using 

carbon-13 (13C), a naturally occurring stable isotope of 

carbon, in conjunction with chemical and mineralogic 

data. These reactions may add carbon that does not 

contain 14C to the dissolved phase or remove carbon 

that may contain 14C from the dissolved phase. For 

example, dissolution, isotopic fractionation, or isotopic 

exchange with carbonate tends to increase δ13C values. 

Although their composition is variable, marine 

carbonates have average values near 0 per mil (Coplen 

and others, 2002) and nonmarine carbonate minerals 

that cement alluvial deposits have average values near 

−4 per mil (Robertson, 1992; Izbicki and others, 1995). 

In contrast, reactions with organic carbon in aquifer 

materials, having values near −21 per mil for marine 

deposits and near −28 per mil for continental deposits, 

tend to decrease δ13C values. 

In complex ground-water systems more than one 

reaction may occur simultaneously and carbon from 

different sources, having different isotopic 

compositions, may contribute to the isotopic 

composition of ground water. Computer programs have 

been developed that use chemical, mineralogic, and 

isotopic data to account for the net effect of different 

reactions on δ13C composition and 14C activity of 

water from wells along a flow path.

t

λ1 2⁄

ln2 ---------- A0Ameas ΣAreact – = ln-------------------------------------
18 Movement and Age of Ground Water in the Western Part of the Mojave Desert, Southern California, USA

Interpretation of Carbon-14 Ages Along Selected Flow Paths

The computer program NETPATH (Plummer 

and others, 1991) was used to calculate the mass 

transfer of constituents between the aquifer and ground 

water, estimate changes in isotopic composition, and 

interpret 14C ages by evaluating the complex reactions 

that control 14C activity. The general approach to 

interpreting the data is summarized in figure 8. Inputs 

to NETPATH include the chemical and isotopic 

composition of water from wells, and chemical 

equations that describe equilibrium reactions and mass 

transfer that occur within the aquifer. Regardless of the 

geochemical setting, the interpretation of 14C ages 

using NETPATH begins with an evaluation of changes 

in major-ion chemistry along the flow path. 

Constituents that have large changes in concentration 

or are especially important to carbonate chemistry, 

such as calcium, need to be included in the 

interpretation. It is desirable to include as many 

constituents as possible to ensure a complete 

representation of ground-water chemistry along the 

flow path. 

Chemical and thermodynamic data are used to 

calculate saturation indexes for minerals within the 

alluvial deposits. Ground water is supersaturated with 

respect to minerals having a saturation index greater 

than zero; these minerals may precipitate but will not 

dissolve. Ground water is undersaturated with respect 

to minerals having a saturation index less than zero; 

these minerals may dissolve but will not precipitate. 

Dissolved-aluminum concentrations were not 

measured as part of this study, and saturation indexes 

for alumino-silicate minerals were estimated by 

assuming equilibrium with the mineral gibbsite 

[Al(OH)3]. Saturation indexes and the potential for 

precipitation or dissolution constrain model results. 

The amount of minerals, gases, or organic material that 

must dissolve, precipitate, or otherwise react with 

ground water at one well to produce the observed 

chemistry at another well farther downgradient is 

calculated. The effect of these reactions on the δ13C 

composition, 14C activity, and age of ground water also 

is calculated. 

A small number of dissolved constituents and 

minerals were included in initial models. Additional 

constituents and minerals were added incrementally 

and incorporated into the interpretation. All masstransfer calculations must conform to thermodynamic 

constraints indicated by saturation indexes. The 

observed δ13C composition was compared with the 

calculated δ13C composition. The reaction set and 

calculated mass-transfers between ground water and 

aquifer materials that minimized the difference 

between observed and calculated δ13C compositions 

along the entire flow path, without violating 

thermodynamic constraints, were used to calculate 14C 

ages. 

Flow Path 1: Surprise Spring Basin 

Ground water in the Surprise Spring subbasin 

within the Morongo basin flows from recharge areas 

near the end of Pipes Wash towards discharge areas at 

Surprise Spring near the Surprise Spring Fault 

(Londquist and Martin, 1991). The Surprise Spring 

Fault is a barrier to ground-water flow, and under 

predevelopment conditions water discharged at land 

surface in this area. Ground-water levels have declined 

more than 30 m as a result of pumping since the 1950s 

and ground water no longer discharges at land surface. 

The flow path is bounded by faults to the north and 

south that also are barriers to flow, and the alluvial 

deposits can be divided into upper and lower aquifers. 

The upper aquifer is composed of material weathered 

from granitic rocks in the San Bernardino Mountains. 

On the basis of x-ray diffraction analysis, quartz, 

plagioclase, and potassium feldspar account for about 

80 percent of the aquifer material. Mica, chlorite, 

calcite, amphibole, amorphous material, and 

montmorillonite and other minerals also are present. 

The lower aquifer is composed of locally derived 

material weathered from surrounding volcanic and 

metamorphic rocks. The upper aquifer is the primary 

water-bearing unit and the wells selected for this flow 

path are completed in the upper aquifer.
Carbon-14 and Carbon-13 19

Chemistry

Thermodynamics

Mass Transfer

Isotopic Composition/

Interpretations

Figure 8. Interpretation of changes in chemistry, thermodynamics, mass transfer, and isotopic composition of water from wells along a flow path.
20 Movement and Age of Ground Water in the Western Part of the Mojave Desert, Southern California, USA

Ground water in the Surprise Spring area has 

dissolved solids concentrations ranging from 177 to 

310 mg/L (table 1). Dissolved-oxygen concentrations 

range from 10.8 mg/L near the recharge area and 

decrease to 1.0 mg/L downgradient. Ground water 

having high concentrations of dissolved oxygen are not 

unusual in alluvial aquifers in arid areas. The pH values 

range from 7.9 to 8.8 and increase with distance 

downgradient. Dissolved-solids, calcium, magnesium, 

bicarbonate (alkalinity), and chloride concentrations 

decrease with distance downgradient. Sodium and 

silica concentrations do not change greatly with 

distance downgradient. 

Mass-transfer calculations were done assuming 

that water from well 3N/6E-16A1 approximated the 

initial chemical and isotopic composition of water 

sampled from wells farther downgradient. The masstransfer calculations account for oxidation of organic 

carbon, precipitation of carbonate minerals, cation 

exchange, and dissolution of primary silicate minerals 

(simulated as dissolution of anorthite) within the 

aquifer (fig. 9). The small decreases in dissolvedoxygen concentrations along the flow path suggest that 

only a small amount of organic matter (simulated as 

having a δ13C composition of −28 per mil) is present 

within aquifer deposits and it oxidizes as water flows 

downgradient. Chemical and thermodynamic data 

suggest that carbonate minerals precipitate primarily as 

calcite (simulated as having a δ13C composition of −4 

per mil) as water flows downgradient. Calculations 

show that smaller amounts of magnesium carbonate 

also precipitate. Calcium and magnesium 

concentrations, and therefore the thermodynamic 

equilibria for carbonate minerals, are affected by the 

exchange of calcium and magnesium for sodium on 

clay-mineral surfaces and to a lesser extent by the 

addition of calcium through the dissolution of 

anorthite. The dissolution of primary silicate minerals, 

such as anorthite, is responsible for the increase in pH 

measured along the flow path. The δ13C composition 

of ground water decreases (becomes more negative) as 

water flows downgradient from −8.8 to −10.2 per mil 

as a result of the oxidation of organic material and 

precipitation of carbonate minerals.

Decreases in the concentration of non-reactive 

constituents, such as chloride or bromide, cannot be 

explained by chemical precipitation of minerals along 

the flow path: chloride and bromide salts are highly 

soluble and thermodynamic data show that they will 

not precipitate. Instead, these data were interpreted as 

reflecting increased aridity of the present-day climate, 

decreased ground-water recharge, and increased 

concentrations of these non-reactive constituents in 

more recently recharged water. This effect was 

simulated in NETPATH through the normalization of 

chloride concentrations to present-day values prior to 

the mass-transfer calculations using the 

evaporation/dilution option within the computer 

program. It was not possible to obtain reasonable masstransfer results, calculated δ13C values, and interpreted 

14C ages without accounting for increased aridity of the 

present-day climate using chloride data. The magnitude 

of the effect suggests that as little as 1,400 to 5,000 

years before present, recharge may have been from

1.3 to 1.6 times greater than under present-day climatic 

conditions. Rather than a uniform desiccation of the 

climate and subsequent decrease in ground-water 

recharge during this period, it is more likely that 

infrequent, extended wet periods occurred in the past. 

Measured carbon-14 activities along the flow 

path ranged from 86 to 41 pmc and decreased with 

distance downgradient (table 1). The initial carbon-14 

activity was very near the initial carbon-14 activity 

estimated for well-leached unsaturated zones 

throughout the Mojave River and the Morongo basins 

(fig. 7). Assuming only radioactive decay and no 

correction for mineralogic reactions, ground-water 

ages along this flow path range from 1,280 to 7,320 

years before present. Interpreted carbon-14 ages for 

water from wells along the flow path through the 

Surprise Spring subbasin range from recent to 5,090 

years before present (table 2). Measured delta carbon13 values and calculated carbon-13 values for water 

from wells along the flow path agree within 1.5 per mil, 

with most values agreeing within 0.7 per mil (table 2). 

Interpreted ages increased in a simple monotonic 

fashion with increasing distance downgradient and are 

almost 30 percent less than uncorrected carbon-14 

ages. 

Simpler interpretations that incorporate only 

oxidation of organic material and precipitation of 

carbonate minerals match measured δ13C values 

poorly, and calculated values are only within ±3.5 per 

mil of the measured values. However, interpreted 14C 

ages differ by no more than 300 years from the 

interpretation shown in figure 9. Inclusion of cation 

exchange improves the match between measured and 

calculated δ13C values only slightly.
Carbon-14 and Carbon-13 21

Table 1. Physical properties, chemical composition, isotopic composition, and saturation indexes, with respect to selected mineral phases, of water from selected wells along flow path 1 through the Surprise Springs subbasin, Morongo ground-water basin, southern California[Saturation indexes greater than 0 indicate supersaturation and the mineral phases would tend to precipitate; saturation indexes less than 0 indicate undersaturation and the mineral would tend to dissolve; mg/L, milligrams per liter; oC, degrees Celsius; dissolved solids is residue on evaporation at 105oC; per mil, parts per thousand; pmc, percent modern carbon]

State well No. Date

Dissolved

oxygen

(mg/L)

pH

Temperature

(oC)

Calcium

(mg/L)

Magnesium

(mg/L)

Potasium

(mg/L)

Sodium

(mg/L)

Bicarbonate

(mg/L)

Chloride

(mg/L)

Sulfate

(mg/L)

Silica

(mg/L)

Dissolved

solids

(mg/L)

3N/6E-16A1 08/11/1994 10.8 7.9 25.0 42 6.5 2.9 47 114 26 38 21 3103N/7E-19N1 08/10/1994 8.6 8.1 25.5 23 2.4 2.4 41 64 20 32 20 2153N/7E-29F1 08/18/1994 6.4 8.1 26.0 17 .5 2.0 41 64 17 29 20 2003N/7E-29R1 07/12/2000 1.0 8.3 25.5 17 .6 .9 44 88 18 29 19 2262N/7E-3A1 08/17/1994 4.0 8.7 31.8 9.9 .2 2.2 48 57 19 31 18 1923N/7E-35P2 08/17/1994 3.9 8.8 28.0 5.4 .1 1.6 49 56 16 25 18 177

State well No.

Delta

oxygen-18

(per mil)

Delta

deuterium

(per mil)

Carbon-14

activity

(pmc)

Delta

carbon-13

(per mil)

Saturation indexes of selected mineral phases

Calcite Gypsum Albite Anorthite Orthoclase Biotite Kaolinite Montmoril- lonite

3N/6E-16A1 −11.0 −78 86 −8.8 0.13 −2.16 −2.34 −4.73 −1.21 3.37 0.79 −0.783N/7E-19N1 −11.5 −81 72 −8.1 −.14 −2.42 −2.58 −5.19 −1.48 2.49 .12 −1.573N/7E-29F1 −11.3 −81 53 −8.2 −.25 −2.57 −2.53 −5.26 −1.52 2.44 .13 −1.553N/7E-29R1 −11.2 −82 53 −7.8 .07 −2.58 −2.60 −5.10 −1.60 2.00 .00 −2.002N/7E-3A1 −11.3 −85 43 −10.4 .12 −2.78 −2.63 −4.97 −1.71 1.39 −1.06 −2.893N/7E-35P2 −11.4 −84 41 −10.2 −.09 −3.11 −2.48 −5.30 −1.66 1.40 −0.99 −2.82
22 Movement and Age of Ground Water in the Western Part of the Mojave Desert, Southern California, USA

–90

–85

–80

–75

–70

–20

–15

–10

–5

0

DELTA DEUTERIUM,

IN PER MIL

Delta

deuterium

Measured

delta carbon-13

0

20

40

60

80

100

0246 8 10 12 14 16 18 20

DISTANCE, IN KILOMETERS

CARBON-14,

IN PERCENT

MODERN CARBON

0

5,000

10,000

Carbon-14

activity Uncorrected

carbon-14 age

–1

0

1

CUMULATIVE

MASS TRANSFER

BETWEEN WELLS,

IN MILLIMOLES

PER LITER

–1

0

1

Organic

carbon Cation

exchange

Total

carbonates

Calculated

delta carbon-13

Interpreted carbon-14 age

16A1

DIRECTION OF GROUND-WATER MOVEMENT

19N1 29F1 29R1 3A1 35P2

DELTA CARBON-13,

IN PER MIL

AGE, IN THOUSANDS OF

YEARS BEFORE PRESENT

CATION EXCHANGE,

IN MILLIMOLES

PER LITER

Primary

silicates

Abbreviated well number

0.01

0.1

1

10

CONCENTRATION,

IN MILLIMOLES

PER LITER

7

8

9

pH, IN STANDARD UNITS

–6

–4

–2

0

2

4

6

SATURATION INDEX

Kaolinite

Anorthite

Calcite

Biotite

pH

Mg2+

Cl -

Ca2+

Na+

HCO3

-

SUPERSATURATED

UNDERSATURATED

Precipitation or Na2 • Clay + (Ca, Mg)+2  2 Na Precipitation or Na + + (Ca, Mg) • Clay 2 • Clay + (Ca, Mg)+2  2 Na+ + (Ca, Mg) • Clay

Dissolution or 2 Na+ + (Ca, Mg) • Clay  Na2 • Clay + (Ca, Mg) Dissolution or 2 Na+ + (Ca, Mg) • Clay  Na +2 2 • Clay + (Ca, Mg)+2

Figure 9. Chemistry, mass transfer, and isotopic composition of water from selected wells along flow path 1 through the Surprise Spring basin, Morongo 

ground-water basin, southern California. Chemical and saturdation index data given in table 1.
Carbon-14 and Carbon-13 23

Table 2. Measured and calculated delta carbon-13 composition, carbon-14 activity, and uncorrected and interpreted carbon-14 ages for water from wells 

along selected flow paths through the Mojave River and the Morongo ground-water basins, southern California

[Per mil, parts per thousand; pmc, percent modern carbon; ybp, years before present; <, less than; —, no data]

State well No.

Measured

delta

carbon-13

(per mil)

Calculated

delta

carbon-13

(per mil)

Measured

carbon-14

activity

(pmc)

Uncorrected

carbon-14

age

(ybp)

Interpreted

carbon-14

age

(ybp)

Flow Path 1

3N/6E-16A1 −8.8 — 86 1,280 <50

3N/7E-19N1 −8.1 −7.6 72 2,590 1,430

3N/7E-29F1 −8.2 −8.9 53 5,200 3,800

3N/7E-29R1 −7.8 −9.2 53 5,310 3,900

2N/7E-3A1 −10.4 −9.1 43 7,000 5,000

3N/7E-35P2 −10.2 −8.8 41 7,320 5,090

Flow Path 2

3N/1W-1R1 −10.6 — 69 3,120 0

4N/1W-26J1 −10.3 −6.8 52 5,410 1,750

4N/1W-19G1 −9.8 −9.8 43 7,030 3,840

4N/2W-9Q1 −11.9 −12.7 38 8,040 4,820

5N/3W-27E1 −10 13.9 36 8,280 4,760

Flow Path 3

4N/7W-33J1 −13.3 — 90 834 0

5N/7W-4D4 −10.6 −7.0 8.6 20,300 18,800

5N/6W-22E2 −10 −9.3 10.7 18,500 16,900

4N/4W-3A2 −9.6 −9.8 4 25,600 25,300

4N/4W-1C2 −9.3 −10.8 .5 43,800 43,000
24 Movement and Age of Ground Water in the Western Part of the Mojave Desert, Southern California, USA

More complex interpretations that incorporate 

clay precipitation and include adjustments for changes 

in natural recharge match measured δ13C values to 

within ±1 per mil—with most values agreeing to within 

±0.4 per mil. The agreement between measured and 

calculated δ13C values is better than the interpretation 

shown in figure 9; however, saturation indexes suggest 

that the water from several downgradient wells along 

the flow path is undersaturated with respect to 

montmorillonite, and precipitation of this mineral is not 

thermodynamically possible. This interpretation may 

not be correct, because montmorillitic clays have a 

wide range of compositions and associated 

thermodynamic data. However, the error is small 

because interpreted 14C ages for calculations that 

include clay precipitation differed by at most 80 years 

from interpreted ages presented in figure 9, with most 

ages differing by less than 20 years.

Flow Path 2: Upper Part of the Mojave River Basin to the East 

of the Mojave River

Ground water in the upper part of the Mojave 

River basin to the east of the Mojave River flows from 

recharge areas in alluvial fans along the flanks of the 

San Bernardino Mountains near the Lucerne Valley in 

the Morongo ground-water basin to discharge areas 

near the floodplain aquifer along the Mojave River. 

Alluvial deposits in the area are composed of material 

weathered from the slopes of the San Bernardino 

Mountains; although the rocks are primarily granitic, 

some limestone crops out in this area. Farther 

downgradient along this flow path, fine-grained basin 

fill and lacustrine deposits are present in the 

subsurface. On the basis of x-ray diffraction data, the 

mineral composition of the aquifer in this area is 

similar to the Surprise Spring flow path except that 

calcite abundance is greatly increased, accounting for 

as much as 20 percent of the material in some samples. 

Material for mineralogic analysis of fine-grained 

lacustrine deposits was not available for this study. 

Ground water along this flow path (fig. 10) has 

dissolved solids that range from 356 to 668 mg/L 

(table 3), dissolved-oxygen concentrations that range 

from 8.0 to <0.2 mg/L and decrease downgradient; and 

pH values that range from 7.5 to 8.0 (table 3). Unlike 

the flow path through the Surprise Spring area, pH 

values initially increase then decrease and are lowest in 

the farthest downgradient well. Dissolved-solids, 

calcium, magnesium, chloride, and sulfate 

concentrations initially decreased and then increased 

with distance downgradient. Because of the increase in 

chloride concentrations along the flow path, it was not 

possible to include climatic effects in the interpretation 

of 14C ages, although these changes almost certainly 

occur and were important in the interpretation of 14C 

data in the Surprise Spring area. Bicarbonate 

concentrations (alkalinity) decreased downgradient. 

Silica concentrations did not change greatly with 

distance downgradient (table 3).

Mass-transfer calculations were done assuming 

that water from well 3N/1W-1R1 approximated the 

initial chemical and isotopic composition of water from 

wells farther downgradient. The calculations account 

for oxidation of organic material, precipitation of 

carbonate minerals, and cation exchange in the same 

manner as in the calculations done along the Surprise 

Spring flow path. The calculations do not account for 

the dissolution of primary silicate minerals, although 

they account for the contribution of solutes from finegrained lacustrine deposits along the downgradient end 

of the flow path.

The measured increase in solutes with distance 

downgradient was assumed to result from the finegrained lacustrine deposits along the downgradient part 

of the flow path and was simulated in the model as 

dissolution of gypsum and halite. Unlike the problem 

considered by Izbicki and others (1995) it was not 

necessary to include dissolution of sodium carbonate 

minerals to explain changes in ground-water chemistry 

measured along this flow path. It is highly unlikely that 

halite, or other chloride containing evaporite minerals, 

are present in lacustrine deposits along this flow path; it 

is more likely that high-chloride water within these 

deposits mixes with fresh ground water. Use of halite is 

an approximation that probably does not reflect the 

complex chemical composition of ground water in the 

lacustrine deposits. As a result of this approximation, 

measured and calculated δ13C values for water from 

wells along the flow path agree only within ±3.5 per 

mil—although measured and calculated values from 

the two wells in the middle of the flow path are in much 

closer agreement (fig.10).
Carbon-14 and Carbon-13 25

–95

–90

–85

–80

–75

–70

–20

–15

–10

–5

0

0

20

40

60

80

100

0 5 10 15 20 25 30 0

5,000

10,000

–6

–4

–2

0

2

4

6

–6

–4

–2

0

2

4

6

Total evaporites

CUMULATIVE

MASS TRANSFER

BETWEEN WELLS,

IN MILLIMOLES

PER LITER

Organic

carbon

Cation exchange

Total carbonates

CATION EXCHANGE,

IN MILLIMOLES

PER LITER

DELTA DEUTERIUM,

IN PER MIL

DELTA CARBON-13,

IN PER MIL

Delta

deuterium

Measured

delta carbon-13

Calculated

delta carbon-13

DIRECTION OF GROUND-WATER MOVEMENT

1R1 26J1 19G1 9Q1 27E1

Abbreviated well number

Carbon-14

activity

Uncorrected carbon-14 age Interpreted carbon-14 age

CARBON-14,

IN PERCENT

MODERN CARBON

AGE, IN THOUSANDS OF

YEARS BEFORE PRESENT

DISTANCE, IN KILOMETERS

Precipitation or Na2 • Clay + (Ca, Mg)+2  2 Na Precipitation or Na2 • Clay + (Ca, Mg) + + (Ca, Mg) • Clay +2  2 Na+ + (Ca, Mg) • Clay

Dissolution or 2 Na+ + (Ca, Mg) • Clay  Na2 • Clay + (Ca, Mg) Dissolution or 2 Na+ + (Ca, Mg) • Clay  Na +2 2 • Clay + (Ca, Mg)+2

Figure 10. Mass transfer and isotopic composition of water from selected wells along flow path 2 through the upper part of the Mojave River groundwater basin east of the Mojave River, southern California. Chemical and saturation index data given in table 3.
26 Movement and Age of Ground Water in the Western Part of the Mojave Desert, Southern California, USA Table 3. Physical properties, chemical composition, isotopic composition, and saturation indexes, with respect to selected mineral phases, of water from selected wells along flow path 2 through the regional aquifer east of the Mojave River, southern California [Saturation indexes greater than 0 indicate supersaturation and the mineral phases would tend to precipitate. Saturation indexes less than 0 indicate undersaturation and the mineral would tend to dissolve. mg/L, milligrams per liter; oC, degrees Celsius; dissolved solids is residue on evaporation at 105oC; per mil, parts per thousand; pmc, percent modern carbon; <, less than] State well No. Date Dissolved oxygen (mg/L) pH Tempera- ture

(

oC)

Calcium

(mg/L)

Magnesium

(mg/L)

Potasium

(mg/L)

Sodium

(mg/L)

Bicarbonate

(mg/L)

Chloride

(mg/L)

Sulfate

(mg/L)

Silica

(mg/L)

Dissolved

solids

(mg/L)

3N/1W-1R1 05/18/1995 8.0 7.6 19.0 59 32 2.7 11 307 6.6 47 33 5084N/1W-26J1 03/30/1994 3.6 7.9 24.5 44 19 2.3 16 202 5.0 28 24 3564N/1W-19G1 04/27/1995 1.6 7.9 21.5 33 14 5.2 78 171 10 160 61 5364N/2W-9Q1 04/27/1995 .5 8 25.0 54 10 2.7 45 115 20 140 34 4275N/3W-27E3 03/10/1994 <.2 7.5 23.8 75 19 3.1 100 107 160 170 29 668 

State well No.

delta

Oxygen-18

(per mil)

delta

Deuterium

(per mil)

Carbon-14

activity

(pmc)

delta

Carbon-13

(per mil)

Saturation indexes of selected mineral phases

Calcite Gypsum Albite Anorthite Ortho- clase Biotite Kaolinite Montmoril- lonite

3N/1W-1R1

−11.8

−85 69

−10.6 0.28

−2.02

−2.30

−4.76

−0.50 4.48 1.82 0.65

4N/1W-26J1

−12

−87 52

−10.3 .38

−2.30

−3.21

−6.22

−1.71 1.07

−.67

−2.26

4N/1W-19G1

−12.2

−88 43

−9.8 .24

−2.11

−1.92

−4.54

−.76 3.77 1.11

−.18

4N/2W-9Q1

−11.6

−84 38

−11.9 .28

−1.57

−1.73

−4.32

−.61 3.67 .95

−.24

5N/3W-27E3

−11.3

−89 36

−10

−.16

−1.43

−1.81

−4.90

−.96 3.82 1.37 .01
Carbon-14 and Carbon-13 27

Measured 14C activities along the flow path 

ranged from 69 to 36 pmc (table 3). Assuming only 

radioactive decay, an initial 14C activity of 100 pmc, 

and no correction for mineralogic reactions, 

uncorrected 14C ages ranged from 3,115 to 8,277 years 

before present (fig. 10). On the basis of the masstransfer calculations presented in this paper, interpreted 

14C ages along this flow path ranged from recent to 

4,820 years before present. The 14C activity of water 

from well 3N/1W-1R1, 63 pmc, was used as the initial 

14C activity for mass-transfer calculations. This value 

is consistent with the initial 14C activity previously 

estimated for poorly-leached or carbonate-rich 

unsaturated zones in the Morongo and the Mojave 

River ground-water basins (fig. 7). Use of the lower 

initial 14C activity resulted in a greater difference 

between the uncorrected 14C age and the interpreted 

age of the ground water than along the Surprise Spring 

flow path.

Simpler interpretations that do not account for 

measured increases in chloride concentrations and 

simulate the contribution of solutes from the finegrained lacustrine deposits solely as the dissolution of 

gypsum match measured and calculated δ13C values 

within ±1.9 per mil. Interpreted 14C ages range from 

recent to 4,820 years before present and are only 

slightly younger than 14C ages that account for 

measured increases in chloride. 

It was not possible to obtain a solution to masstransfer calculations and interpretations of groundwater ages along the study flow path if more 

complicated calculations that include dissolution of 

silicate minerals and (or) precipitation of clay minerals 

were included. Although these reactions probably 

occur, their effect on pH and other constituents is 

masked by the contributions of solutes from the 

lacustrine deposits. Results of calculations done along 

this flow path are poorly understood and that the 

measured changes are not accurately simulated by the 

dissolution of mineral phases.

Flow Path 3: Upper Part of the Mojave River Basin to the 

West of the Mojave River 

Ground water in the upper part of the Mojave 

River basin to the west of the Mojave River flows from 

recharge areas in the alluvial fans along the flanks of 

the San Bernardino Mountains to discharge areas near 

the Mojave River. As a result of tectonic activity near 

Cajon Pass and subsequent erosion, the alluvial fans 

are connected to the San Bernardino Mountains only in 

the western part of the study area near Sheep Creek, 

and ground water in the regional aquifer flows radially 

outward from this area. Alluvial deposits in this part of 

the basin are composed of material weathered from the 

granitic and metamorphic rocks in the San Bernardino 

Mountains. Fine-grained basin-fill and lacustrine 

deposits also are present in this area farther from the 

mountain front; however, these deposits were not 

encountered along the flow path presented. On the 

basis of x-ray diffraction data, the mineral composition 

of the aquifer in this area is similar to that of the 

Surprise Spring flow path except that mica, chlorite, 

amorphous material and calcite were more abundant. 

However, calcite was not as abundant as in deposits 

along the flow path east of the Mojave River.

 Many of the wells sampled along this flow path 

are observation wells having short screen lengths 

(typically about 6 m) completed in deeper parts of the 

aquifer. Changes in the chemical and isotopic 

composition of water from wells along this flow path 

were studied previously by Izbicki and others (1995) 

and Kulongoski and others (in press).

Dissolved-solids concentrations ranged from 239 

to 736 mg/L and decreased downgradient. Dissolved 

oxygen was not measured in sampled wells near the 

mountain front but was present in water from wells 

farther downgradient. The pH values ranged from 

7.3 to 9.6 and increased with distance downgradient. In 

general, calcium, magnesium, potassium, chloride, and 

sulfate concentrations decrease downgradient. 

Bicarbonate (alkalinity) concentrations decrease and 

then increase with distance downgradient. In contrast 

to previous flow paths, the δ13C composition of water 

along this flow path became less negative with distance 

downgradient.

Mass-transfer calculations were done assuming 

that water from well 4N/7W-33J1 approximated the 

initial chemical and isotopic composition of water from 

wells farther downgradient. The calculations account 

for oxidation of organic carbon, precipitation of 

carbonate minerals, cation exchange, and dissolution of 

primary silicates in the same manner as calculations 

along the other flow paths. In addition, it was possible 

to incorporate precipitation of secondary silicate 

minerals, simulated as montmorillonite. 
28 Movement and Age of Ground Water in the Western Part of the Mojave Desert, Southern California, USA

Carbon-14 activities range from 90 to 0.5 pmc 

(table 4). Assuming only radioactive decay and no 

correction for mineralogic reactions, uncorrected 14C 

ages range from 834 to 43,800 years before present 

(fig. 11). Interpreted 14C ages range from recent to 

43,000 years before present but are subject to great 

uncertainty and exceed the practical range of the dating 

technique of 20,000 years before present. The great age 

of water from wells along this flow path is a function of 

three factors: small recharge rates, aquifer geometry 

(ground-water movement is radially away from a single 

recharge location), and the deep short-screened wells 

sampled along this section. This last factor results in 

data that contrasts with data collected from longscreened production wells that integrate larger volumes 

of water than the short screened observation wells.

Simpler interpretations that included only 

oxidation of organic matter and precipitation of 

carbonate minerals yielded δ13C values that differed by 

2 to 3 per mil from measured values. Four of the five 

interpreted 14C ages calculated on the basis of these 

simpler interpretations were within 100 years of the 

more complex interpretation shown in figure 11. The 

greatest difference in interpreted age was about 800 

years. Given the great age of ground water along this 

flow path, a difference in estimated ground-water age 

of 800 years may not be important.

Limitations on Carbon-14 Interpretations

Carbon-14 ages calculated for ground water are 

interpretive and subject to considerable uncertainty. 

Davis and Bentley (1982) estimated that, for aquifers 

where chemistry is well understood, interpreted 14C 

ages are within ±20 percent of the actual age. For areas 

where chemistry is less well understood, Davis and 

Bentley (1982) estimated that the error may be as great 

as ±100 percent of the actual age. Results of this study 

show that although the calculated δ13C composition of 

ground water along the flow path is sensitive to 

chemical reactions simulated by the mass-transfer 

calculations done using NETPATH, the interpreted 14C 

ages are not as sensitive. As a result, interpreted 

carbon-14 ages estimated on the basis of a reasonable, 

but simple, representation of ground-water 

geochemistry may be sufficiently accurate for most 

purposes. 

In many studies the greatest source of 

uncertainty in interpreting 14C data is in estimating the 

initial chemistry and isotopic composition of the 

recharge water. This is a common problem in areas 

where present-day recharge is absent or very small. In 

this study, the initial chemistry and isotopic 

composition of recharge water was estimated through 

sample collection. In some areas, such as in the 

Surprise Spring area, the results compared well with 

theoretical values and measured values in other 

aquifers. In areas where large amounts of carbonate 

minerals were present, initial 14C activities of 

recharging ground-water were significantly lower—but 

the actual composition of water that recharged the 

aquifer thousands of years ago can not be precisely 

known. This type of error is most significant in the 

estimate of the absolute age of water from a well and is 

less important in estimates of the time-of-travel from 

one well to another farther downgradient (Izbicki and 

Martin, 1998; Zhu and Murphy, 2000).

Another source of uncertainty in estimating 

ground water age is that 14C production varies with 

solar activity and has not been constant during the last 

24,000 years (Stuiver and others, 1998). Changes in 

14C production as a result of changes in solar activity 

during recent geologic time were not factored into 

ground-water ages estimated as part of this study. 

DISCUSSION

Tritium and 14C have been used to estimate 

ground-water recharge, ages, and travel times in 

aquifers throughout the southwestern United States and 

in other parts of the world. In most of these studies, the 

application of the environmental tracers has been 

restricted to ground water along simplified flow paths 

from recharge areas to discharge areas. The areal 

distribution of ground-water ages and the complex 

interaction of ground water between riparian aquifers 

and surrounding basin-fill aquifers, such as those found 

in the Mojave River ground-water basin, have not been 

routinely considered.
Discussion 29

Table 4. Physical properties, chemical composition, isotopic composition, and saturation indexes, with respect to selected mineral phases, of water from selected wells along flow path 3 through the 

regional aquifer west of the Mojave River, southern California

[Saturation indexes greater than 0 indicate supersaturation and the mineral phases would tend to precipitate; saturation indexes less than 0 indicate undersaturation and the mineral would tend to dissolve; mg/L, milligrams per liter; oC, degrees Celsius; dissolved solids is residue on evaporation at 105oC; per mil, parts per thousand; pmc, percent modern carbon; —, no data]

State well No. Date

Dissolved

oxygen

(mg/L)

pH

Temperature

(oC)

Calcium

(mg/L)

Magnesium

(mg/L)

Potasium

(mg/L)

Sodium

(mg/L)

Bicarbonate

(mg/L)

Chloride

(mg/L)

Sulfate

(mg/L)

Silica

(mg/L)

Dissolved

solids

(mg/L)

4N/7W-33J1 06/08/93 — 7.3 15.5 120 35 5.5 15 376 6.2 160 15 7365N/7W-4D4 05/27/93 — 8.6 29.5 19 2.4 3.4 80 71 3.4 160 15 3555N/6W-22E2 06/01/94 3.4 8.2 26.4 9.2 1.6 2.8 62 83 3 86 15 2664N/4W-3A2 03/09/94 1.1 9.6 21.5 .9 .09 .5 68 171 1.9 2.3 19 2704N/4W-1C2 01/13/93 .5 9.6 17.5 .89 .06 .6 31 171 5.1 11 16 239

State well No.

delta

Oxygen-18

(per mil)

delta

Deuterium

(per mil)

Carbon-14

activity

(pmc)

delta

Carbon-13

(per mil)

Saturation indexes of selected mineral phases

Calcite Gypsum Albite Anorthite Orthoclase Biotite Kaolinite Montmoril- lonite

4N/7W-33J1 −11.6 −80 90 −13.3 0.26 −1.28 −3.14 −5.44 −1.12 4.45 1.84 0.165N/7W-4D4 −11.8 −84 8.6 −10.6 .25 −1.92 −1.42 −2.72 −.51 5.12 1.40 −.115N/6W-22E2 −11.8 −85 10.7 −10 −.37 −2.42 −1.82 −3.94 −.84 4.78 1.53 −.114N/4W-3A2 −11.9 −86 4 −9.6 .03 −5.05 −.50 −2.66 −.25 5.40 1.55 .114N/4W-1C2 −11.1 −80 .5 −9.3 .01 −4.35 −.96 −3.09 −.24 5.33 1.50 −.02
30 Movement and Age of Ground Water in the Western Part of the Mojave Desert, Southern California, USA

0

20

40

60

80

100

0 5 10 15 20 25 30 35 40

DISTANCE, IN KILOMETERS

0

10,000

20,000

30,000

40,000

50,000

–100

–90

–80

–70

–20

–10

0

–6

–4

–2

0

2

4

6

–6

–4

–2

0

2

4

6

33J1 4D4 22E2 3A2 1C2

DIRECTION OF GROUND-WATER MOVEMENT

Abbreviated well number

CUMULATIVE

MASS TRANSFER

BETWEEN WELLS,

IN MILLIMOLES

PER LITER

CATION EXCHANGE,

IN MILLIMOLES

PER LITER

DELTA DEUTERIUM,

IN PER MIL

DELTA CARBON-13,

IN PER MIL

CARBON-14,

IN PERCENT

MODERN CARBON

AGE, IN THOUSANDS OF

YEARS BEFORE PRESENT

Carbon-14 activity

Uncorrected carbon-14 age

Calculated carbon-14 age

Calculated

delta carbon-13

Measured

delta carbon-13

Delta

deuterium

Cation exchange

Total

carbonates

Secondary

silicates

Primary

silicates

Organic

carbon

Precipitation or Na2 • Clay + (Ca, Mg)+2  2 Na Precipitation or Na + + (Ca, Mg) • Clay 2 • Clay + (Ca, Mg)+2  2 Na+ + (Ca, Mg) • Clay

Dissolution or 2 Na+ + (Ca, Mg) • Clay  Na2 • Clay + (Ca, Mg) Dissolution or 2 Na+ + (Ca, Mg) • Clay  Na +2 2 • Clay + (Ca, Mg)+2

–15

–5

Figure 11. Mass transfer and isotopic composition of water from selected wells along flow path 3 through the upper part of the Mojave River ground-water 

basin west of the Mojave River, southern California. Chemical and saturation index data given in table 4.
Discussion 31

Altitudes of the San Gabriel and the San 

Bernardino Mountain ranges control the distribution of 

ground-water recharge to aquifers in the Mojave River 

and the Morongo ground-water basins (Izbicki and 

others, 1995). Low-altitude gaps in these mountain 

ranges allow cool, moist air masses to enter the Mojave 

Desert and produce precipitation. For example, winter 

precipitation near Cajon Pass gives rise to stormflows 

in the Mojave River that recharge the floodplain aquifer 

along the entire length of the river as far east as Afton 

Canyon, 160 km from the mountain front. In most 

areas, ground water in the floodplain aquifer contains 

tritium and was recharged after 1952. However, the 14C 

activity ground water in the floodplain aquifer in the 

Mojave Valley suggests that this water has great age. 

The surrounding and underlying regional aquifer is 

recharged by infiltration of streamflow from the higher 

altitudes of the San Gabriel and the San Bernardino 

Mountain ranges. Most water in the regional aquifer 

recharged many thousands of years ago, and recently 

recharged water containing tritium is found only in 

canyons and along alluvial fans near the mountain 

front. Under predevelopment conditions, ground water 

in the regional aquifer discharged to the floodplain 

aquifer and ultimately to the Mojave River.

Although the aquifers are hydraulically 

connected, in most areas the isotopic composition of 

ground water in the two aquifers is distinct. Izbicki and 

others (1995) observed sharp contrasts in the δ18O and 

δD composition and large differences in the 14C 

activity of water in the two aquifers were observed. The 

oldest ground water sampled was from deep 

observation wells at the downgradient ends of long 

flow paths through the regional aquifer near where 

under predevelopment conditions ground water 

discharged to the floodplain aquifer. Movement of 

water between aquifers occurred near faults that acted 

as barriers to ground-water flow. On the upgradient 

side of these faults, ground water from the regional 

aquifer discharged to the floodplain aquifer. On the 

downgradient side of the faults, ground water from the 

floodplain aquifer recharged the regional aquifer and 

underlying deposits. Detailed sample collection from 

depth-dependent observation wells is required to 

observe the complex exchange of water between the 

floodplain aquifer and the surrounding and underlying 

regional aquifer systems.

An inverse mass-balance modeling approach 

using the computer program NETPATH (Plummer and 

others, 1991) was used to interpret 14C activity data 

and estimate ground water ages in the Mojave River 

and the Morongo ground-water basins. The flow path 

through the Surprise Spring area of the Morongo 

ground-water basin is representative of reactions that 

occur in relatively non-reactive desert aquifer systems, 

characterized by an absence of organic material and 

increases in pH as a result of primary silicate 

weathering. The effects of gradual desiccation resulting 

from decreasing recharge were required to interpret 

changes in ground-water chemistry measured in this 

simple geochemical system. These effects were masked 

along the other study flow paths by uncertainty in the 

estimates of the initial 14C activity of recharging 

ground water, and by the dissolution of soluble 

minerals along the study flow paths, but the effects 

almost certainly were present. Changes in rechargewater chemistry with time have probably occurred for 

aquifers throughout the southwestern United States and 

in other arid areas; however, these changes are not 

usually incorporated into the interpretation of 14C data. 

Corrections to measured 14C activity data were 

most important in estimates of ground water ages less 

than 10,000 years before present. In this range, 

interpreted 14C ages were about 30 percent lower than 

uncorrected ages estimated assuming an initial 14C 

activity of 100 pmc and neglecting rock-water 

reactions (fig. 12). Corrections to measured 14C 

activity data for mineralogic reactions are greater in 

areas having higher dissolved-solids concentrations. In 

general, interpreted ages in areas having high-dissolved 

solids concentrations are probably less accurate than 

interpreted ages in areas having low dissolved-solids 

concentrations. 
32 Movement and Age of Ground Water in the Western Part of the Mojave Desert, Southern California, USA

0

0 10,000

0

0

5,000

10,000

20,000

30,000

40,000

50,000

10,000 20,000

UNCORRECTED CARBON-14 AGE,

IN YEARS BEFORE PRESENT

INTERPRETED CARBON-14 AGE,

IN YEARS BEFORE PRESENT

30,000 40,000 50,000

5,000

EXPLANATION

1:1 Correlation line

Carbonate poor

Carbonate rich

10,000

1:1 Correlation line

Figure 12. Comparison of uncorrected carbon-14 ages and interpreted carbon-14 ages for ground water in the Mojave River and the Morongo groundwater basins, southern California.
References 33

In relatively non-reactive alluvial aquifers, such 

as those studied within the western Mojave Desert 

ground water having low 14C activity tended to have 

great age regardless of the corrections applied during 

the interpretation of the data (fig. 12). However, results 

of this study show that recently recharged water that 

has passed through thick unsaturated zones beneath 

intermittent streams may have low 14C activity. 

Similarly, recently recharged water in strongly 

reducing coastal aquifers in southern California can 

have 14C activities as low as 14 pmc (Izbicki and 

others, 1992). In both cases the water can be readily 

identified on the basis of unique chemical composition. 

Care must be taken to ensure that the hydrologic setting 

and ground-water chemistry are sufficiently well 

characterized that appropriate, reasonable reactions are 

used to interpret 14C activity data and estimate groundwater age.

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